amdgpu: don't dereference a NULL resource in sysfs code

dma_resv_trylock being successful doesn't guarantee that bo->tbo.base.resv
is not NULL, so check its validity before using it.
Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_object.c

@@ -1601,7 +1601,9 @@ u64 amdgpu_bo_print_info(int id, struct amdgpu_bo *bo, struct seq_file *m)
 	u64 size;
 
 	if (dma_resv_trylock(bo->tbo.base.resv)) {
-
+		if (!bo->tbo.resource) {
+			placement = "NONE";
+		} else {
 			switch (bo->tbo.resource->mem_type) {
 			case TTM_PL_VRAM:
 				if (amdgpu_res_cpu_visible(adev, bo->tbo.resource))
@@ -1632,6 +1634,7 @@ u64 amdgpu_bo_print_info(int id, struct amdgpu_bo *bo, struct seq_file *m)
 				placement = "CPU";
 				break;
 			}
+		}
 		dma_resv_unlock(bo->tbo.base.resv);
 	} else {
 		placement = "UNKNOWN";